Secondary batteries generate electric energy through electrochemical oxidation and reduction reaction and are widely used for various purposes. The use range of the secondary batteries gradually expands. For example, the secondary batteries are used for apparatuses carried by a human hand such as mobile phones, lap-top computers, digital cameras, video cameras, tablet computers, and motor-driven tools, various electrically-driven power apparatuses such as electric bicycles, electric motorcycles, electric automobiles, hybrid automobiles, electric ships, and electric airplanes, electric power storage apparatuses used for storing power generated through new regeneration energy or surplus generated power, and uninterruptable power systems for stably supplying power to various kinds of information communication apparatuses including server computers and base stations for communication.
A charging method of a secondary battery varies depending on the purpose of application. Miniaturized secondary batteries used for hand-held terminals such as smartphones or mobile phones are charged by separate chargers. Also, large-scale secondary batteries used for electrically-driven power apparatuses are charged by power produced by the power apparatuses themselves. For example, large-scale secondary batteries mounted on electric automobiles or hybrid automobiles are charged by power produced by generators connected to engines or regeneration power produced when the automobiles reduce their speeds.
A secondary battery generates heat while it is charged. Most of heat generated during a charging process is Joule's heat generated by an internal resistance of the secondary battery. The heat may be emitted to the outside of the secondary battery through conduction or convection. However, when heat generated from the secondary battery is greater than heat emitted to the outside, heat is continuously accumulated inside the secondary battery and thus the temperature of the secondary battery rises continuously.
The life of the secondary battery is reduced when the temperature of the secondary battery excessively rises. Overheating of the secondary battery deteriorates properties of a kind of chemicals participating in an electrochemical reaction or causes physical transformation of a polymer material inside the battery vulnerable to heat. For example, when the secondary battery is overheated, a porous polymer separation layer between an anode and a cathode is softened and some of pores are closed, which may raise an internal resistance of the secondary battery. The rise of the internal resistance acts as a primary factor which reduces a charging capacity of the secondary battery.
Therefore, when the temperature of the secondary battery exceeds a threshold temperature set in advance, an appropriate management for lowering the temperature is required. The temperature of the secondary battery may be controlled through a cooling mechanism. For example, a water-cooled or air-cooled cooling device is coupled to the secondary battery, and when the temperature of the secondary battery is excessively raised, the temperature of the secondary battery may be reduced to an appropriate level by operating the coupled cooling device.
However, in the case where a space of a device or an apparatus on which the secondary battery is mounted is narrow, the cooling device cannot be coupled to the secondary battery. For example, when a secondary battery is mounted on a small electric automobile, a narrow space is provided for the installation of the secondary battery and a weight increase should be minimized by the mounting of the secondary battery. Thus, it is difficult to couple the secondary battery to the cooling device occupying a volume and weight of some degree.
For the secondary batteries used under the above circumstances, structural design technology for effectively emitting heat generated during a charging process through atmosphere by increasing the area of the secondary battery exposed to air is required.
However, there is a limit in reducing the temperature of the secondary battery to an appropriate level with only the structural design of the secondary battery. Even when heat generated from the secondary battery is emitted to the atmosphere, a circumstance which causes heat accumulation may occur.
Therefore, the inventor of the present disclosure has recognized that additional complementary technology of dynamically varying a charging condition such that the temperature of the secondary battery is maintained at an appropriate level during a process of charging the secondary battery is required.